I am writing this right after I was just about to pull my hair out due to the fact that I rely on my N900 as my primary phone (which is not necessarily the greatest idea if you tinker with it quite a bit).

I have noticed that after updating to this kernel:

Linux N900 2.6.28.10-power50

produces an issue with the bleeding-edge wireless driver that allows the N900 to be able to do packet injection. If you try to enable the driver and use it, the wlan0 interface will disappear. You will then have to reboot your phone to be able to get the wlan0 interface back.

So out of curiosity I decided to try rolling back to the previous kernel I was using that was provided with the  bleeding-edge drivers. Case and point, this was a BAD IDEA. The installation failed and upon rebooting my N900, the N900 went into a reboot loop and to power the phone off I had to pull the battery.

Gladly, I was able to reflash the phone and get it functioning.

Conclusion:

If you want to be able to do packet injection (and use awesome tools like reaver and aircrack) on your N900, you MUST (for now) use the kernel (kernel-power_2.6.28-maemo46-wl1) from  bleeding-edge.

Feel free to contact me if you need any help regarding this and I will do my best to help you.